Thermally integrated fuel cell stack

ABSTRACT

A thermally integrated fuel cell system includes a stack zone, a burner zone and a low temperature zone. The fuel is combined with steam and passed sequentially through a primary reformer and a secondary reformer or a radiative fuel heat exchanger. Air may be passed sequentially through an afterburner heat exchanger and a radiative air heat exchanger such that the radiative heat exchanger may be used to heat the stack zone. The stack exhaust is combusted in an afterburner. Afterburner exhaust heats the primary reformer, the high temperature air heat exchanger, the low temperature air heat exchanger and steam generator. The stack zone includes the stacks, the secondary reformer and the radiative heat exchanger. The burner zone includes the afterburner which includes a start burner, the primary reformer and the high temperature air heat exchanger. The low temperature zone includes the low temperature air heat exchanger and a steam generator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT Patent application No.PCT/CA2004/000681 filed on May 6, 2004, designating the United Stateswhich is a continuation of U.S. patent application Ser. No. 10/249,772filed on May 6, 2003.

BACKGROUND OF INVENTION

The present invention relates to a thermally integrated high temperaturefuel cell system.

High temperature fuel cells such as solid oxide fuel cells comprise anelectrolyte sandwiched between a cathode and an anode. Oxygen reactswith electrons at the cathode to form oxygen ions, which are conductedthrough the ion-conducting ceramic electrolyte to the anode according tothe reaction:½O₂+2e⁻→O²⁻  (1)

At the anode, oxygen ions combine with hydrogen and carbon monoxide toform water and carbon dioxide thereby liberating electrons according tothe exothermic reactions:H₂+O²⁻→H₂O+2e⁻  (2)CO+O²⁻→CO₂+2e⁻  (3)

In conventionally-designed solid oxide fuel cells, the aboveelectrochemical reactions usually take place at temperatures of betweenabout 600° C. and 1000° C. Therefore, thermal management is an importantconsideration in the design of fuel cell systems. SOFC stacks producehigh grade heat and it would obviously improve the overall efficiency ofthe operation if that high grade heat could be captured and utilized.

Typically, incoming fuel and air streams are preheated both duringstartup when the stack is at an ambient temperature and during operatingconditions when the stack is at an elevated temperature. It is wellknown to use heat exchangers to extract heat from the stack exhausts,and use that heat to preheat incoming gas streams.

In PCT Application No. PCT/US02/12315 (W002/087052), a waste energysubassembly is provided which includes a combustion zone and a heatexchanger. A separate reformer subassembly provides reformate to thecombustion zone where it is combusted to heat the system. Once atoperating conditions, the stack exhaust is combusted in the combustionzone and heat is transferred to the incoming air and reformate streamsin the heat exchanger. In Applicant's co-pending PCT Application No.CA01/01014, an integrated module is described which is associated with afuel cell stack and includes an afterburner, a fuel reformer and a heatexchanger. The afterburner burns unused fuel in the fuel cell exhauststreams and heats the fuel reformer and an incoming cathode air stream.

It is a goal of both of these technologies to thermally integrate thefuel cell system and some thermal integration is achieved. However,further integration and better efficiencies may be achievable.

Therefore, there is a need in the art for a thermally integrated fuelcell system.

SUMMARY OF INVENTION

The present invention provides a thermally integrated fuel cell system.In one aspect of the invention, the invention may comprise a fuel cellsystem including a fuel cell stack producing an anode exhaust stream anda cathode exhaust stream, said system comprising: (a) a fuel supplyassembly for supplying reformate fuel to the stack comprising: i. a fuelsupply, ii. a water heat exchanger for generating steam, iii. a primaryreformer, and iv. a radiative fuel heat exchanger; (b) an air supplyassembly for supplying air to the stack comprising: i. a low temperatureair heat exchanger, ii. a high temperature air heat exchanger, and iii.a radiative air heat exchanger; (c) an afterburner which receives theanode and cathode exhaust streams from the fuel cell stack and combuststhe exhaust streams to produce a combustion stream; (d) wherein thecombustion stream provides heat energy to the primary reformer, the hightemperature air heat exchanger, the water heat exchanger and the lowtemperature air heat exchanger; and (e) wherein the radiative air heatexchanger and radiative fuel heat exchanger each receive radiative heatenergy from the stack.

In one embodiment, the radiative fuel heat exchanger may be a secondaryreformer where additional fuel reforming takes place. The air supplyassembly may comprise a first air stream which passes through the hightemperature air heat exchanger and a second air stream which passesthrough the radiative air heat exchanger, wherein the first air streamand second air stream combine downstream from the radiative air heatexchanger. The system may additionally comprise an equalization heatexchanger which receives a reformate supply, the first air stream andthe second air stream and outputs a temperature equalized reformatestream and an air stream to the stack.

In an alternative embodiment, the air supply assembly may comprise anair stream which passes through the high temperature heat exchanger andthe radiative air heat exchanger in series. Optionally, the air supplyassembly may further comprise a bypass air stream which does not passthrough the high temperature heat exchanger. As well, the fuel supplyassembly may further comprise a bypass fuel stream which does not passthrough the fuel reformer thereby adjusting effectively the amount ofreformer reforming.

In one aspect of the invention, the invention may comprise a fuel cellsystem comprising a fuel cell stack and further comprising: (a) an airsupply for providing air to the stack; (b) a fuel supply including afuel reformer for providing reformate fuel to the stack; (c) anafterburner which burns raw fuel or unused fuel in an anode exhauststream, or both raw fuel and unused fuel in the anode exhaust stream andwhich comprises a start burner; (d) an afterburner heat exchanger fortransferring heat from the afterburner to the air supply and the fuelsupply; and (e) a radiative heat exchanger for exchanging heat betweenthe fuel cell stack and the air supply or the fuel supply, or both theair supply and the fuel supply, primarily by radiation.

In one embodiment, the air supply may comprise a first air stream whichreceives heat from the afterburner heat exchanger and a second airstream which receives heat from the radiative heat exchanger, whereinthe first air stream and the second air stream combine at a pointdownstream from the afterburner heat exchanger and the radiative heatexchanger. In an alternative embodiment, the air supply may pass throughthe afterburner heat exchanger and then through the radiative heatexchanger, and the fuel supply may pass through the afterburner heatexchanger and then through the radiative heat exchanger.

In another aspect, the invention may comprise a method of heating a fuelcell stack on startup comprising the steps of: (a) operating the startburner and/or afterburner to heat the high temperature air heatexchanger; (b) directing heated air from the high temperature air heatexchanger to the radiative heat exchanger; and (c) heating the stackwith the radiative heat exchanger substantially by radiative means.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of an exemplary embodimentwith reference to the accompanying simplified, diagrammatic,not-to-scale drawings. In the drawings:

FIG. 1 is a schematic representation of one embodiment of the presentinvention;

FIG. 2 is a schematic representation of one embodiment of the presentinvention.

FIG. 3 is schematic representation of one alternative embodiment of thepresent invention.

FIG. 4 is a cross-sectional schematic showing the different zones of asystem of the present invention.

FIG. 5 is a cross-sectional detail of the stack zone of one embodiment.

DETAILED DESCRIPTION

The present invention provides for a thermally integrated fuel cellsystem. When describing the present invention, all terms not definedherein have their common art-recognized meanings.

Generally, a system of the present invention achieves thermalintegration primarily by exchanging heat from the exhaust side to theintake side of the fuel cell system in order to preheat the intakestreams. In addition, radiative heat from the fuel cell stack itself iscaptured, or the stack may be heated by radiative means. The features ofthe invention described herein may enhance the efficiency orcontrollability of the system.

FIG. 1 shows a generalized schematic representation of the high gradeheat sources and heat sinks of the present invention. The solid arrowsrepresent heat exchange and not fluid flows. The dashed line arrowsrepresent fluid flows. The system includes two heat sources, which arethe fuel cell stack (10) itself, and an afterburner (12). The systemalso includes two heat sinks, which are the process air stream (14) andthe process fuel stream (16). The latter includes a fuel reformer. Asdepicted in FIG. 1, each of the two heat sinks receives heat from bothheat sources. In one embodiment, the process air stream (14) is heatedby an afterburner and is also heated by the stack.

In one embodiment depicted in FIG. 1, the process air stream may beseparated into two separate parts which flow in parallel. Therefore, onepart (14A) of the process air stream receives heat primarily from thestack, while a second part (14B) of the process air stream receives heatprimarily from the afterburner. The two parts may then be recombinedbefore entering the stack (10). In another embodiment, the process airstream may be split in series rather than in parallel. The process fuelstream (16) receives heat from the afterburner and also from the stackbut is not necessarily split into parallel flows. The split of theprocess fuel stream may be conceived of as two serial portions. Thisdivision of the heat sinks permits greater controllability of thethermal integration of the system. The control system may divert greaterflow through a first part of the air heat sink from the second part,which will have the effect of increasing cooling of the heat sourceassociated with first part of the air flow.

An embodiment where the first and second parts of the process air streamflow in parallel may be more suitable for a high temperature fuel cellstack with little to no internal fuel reforming capability. Preferably,there is no direct thermal link between the two heat sources. Morepreferably, the two heat sources are isolated from each other withthermal insulation, thus maintaining the separate thermal availabilityof either heat source.

The split of the two heat sinks, the process air stream and the processfuel stream, into two parts, one part of which is thermally linked tothe stack as a high grade heat source, and another part of which isthermally linked to the afterburner as the other high grade heat source,permits greater controllability of the system and may provide for ahigher degree of thermal efficiency. For example, the stack may becooled by directing a greater proportion of the process air streamthrough the part which is thermally linked to the stack. In anotherexample, if greater fuel utilization results in less heat produced bythe afterburner, more heat from the stack may be utilized by the heatsinks. These and other control strategies will be apparent to thoseskilled in the art.

Therefore, in one embodiment of the present invention shownschematically in FIG. 2, a fuel cell system comprises a process fuelsource (20) and a process air source (22) both of which may initially beat ambient temperatures. The fuel stream (20) is combined with steam(24) generated by a steam generator (26) or a water heat exchanger. Inan alternative embodiment, the fuel may be mixed with water prior toentering into the steam generator (26) or water heat exchanger, or mixedwithin the heat exchanger itself. The combined steam/fuel stream (28)then enters a primary reformer (30) which reforms the fuel to hydrogenor a mixture of hydrogen and carbon monoxide (syngas) using anyreforming process such as catalytic steam reforming, or partialoxidation reforming or some combination of these reforming processes. Inone embodiment, the primary reformer is a steam reformer implementingsteam reformation of a hydrocarbon such as natural gas. The reformatestream (32) then passes into a secondary reformer (34) where additionalreforming to hydrogen or syngas takes place, leveraging radiative heatfrom the stack when needed. The secondary reformer may implement thesame or different reforming process as the primary reformer.

The secondary reformate stream (35) may then be passed through anoptional equalization heat exchanger (36) which serves to equalize thetemperature between the reformate stream (35) and the heated air stream(48) before entering the fuel cell stack (10) and which also receivessome heat energy from the stack (10). The equalization heat exchanger isan optional component and may assist in minimizing any temperaturegradients between the stack and the incoming reactants.

The air source (22) initially passes into a low temperature air heatexchanger (38) where it is split into two streams. A valve (39) ordiverter, or independent valves, may direct air equally into bothstreams, into one stream to the exclusion of the other, or direct agreater proportion of air into one stream. It is preferable to split theair stream prior to entry into the low temperature air heat exchanger soas to avoid the need for high temperature materials for the valves (39).A first air stream (40) passes to a high temperature air heat exchanger(42) while a second air stream (44) goes on to a radiative heatexchanger (46). The two streams from the high temperature air heatexchanger (42) and the radiative heat exchanger (46) are then combinedinto a heated air stream (48) before entering the equalization heatexchanger (36) or the stack itself (10).

Once reacted in the stacks, the cathode exhaust (50) and the anodeexhaust (52) pass into an afterburner (54) where any residual fuel inthe exhaust stream is combusted. The cathode and anode exhausts arecombined at this stage, and the cathode exhaust preferably providessufficient residual oxygen necessary for combustion of any remainingfuel in the afterburner (54). The hot afterburner exhaust (56) is usedfirst to heat the fuel input stream in the primary reformer (30) and theair input stream in the high temperature air heat exchanger (42). In oneembodiment, the afterburner (54) is combined with the primary reformer(30) and the high temperature air heat exchanger in an afterburner heatexchanger (54) which may be an integrated module (not shown). Anexemplary integrated module is illustrated and described in U.S. Pat.No. 6,749,958, issued Jun. 15, 2004 and entitled “Integrated Module forSolid Oxide Fuel Cell Systems”, the contents of which are incorporatedherein by reference. As used herein, an “afterburner heat exchanger” isintended to refer to any heat exchanger which extracts heat from theafterburner or the afterburner exhaust. Therefore, both the primaryreformer (30) and the high temperature air heat exchanger (42) may beconsidered part of an afterburner heat exchanger.

Upon exiting the afterburner heat exchanger, the hot exhaust stream (56)may then be used to heat the steam generator (26) or water heatexchanger in the fuel supply and the low temperature air heat exchanger(38) in the air supply. The exhaust stream may be split to heat theseheat exchange elements, or they may be heated in series.

In one embodiment, the secondary reformer (34) and the radiative heatexchanger (46) are positioned to receive radiative energy (R) from thestacks (10). The heat from the stacks is available from the inefficiencyin the electrochemical stack reactions.

The afterburner may also include a start burner (not shown), or startburner capability, which burns unreformed or raw fuel and exhausts intothe same exhaust as the afterburner (56).

In an alternative embodiment of the present invention, as shownschematically in FIG. 3, a fuel cell system may comprise a process fuelsource (220) and a process air source (222) both of which may be atambient temperatures. The fuel stream (220) is combined with steam (224)generated by a water heat exchanger (226) or a steam generator. Thecombined steam/fuel stream (228) then enters a primary reformer (230).In one embodiment, the primary reformer is a steam reformer whichreforms natural gas into syngas. The reformate stream (232) then passesinto a radiative fuel heat exchanger (234) where the reformate stream isheated by radiation from the fuel cell stacks. The reformate stream thenenters the fuel cell stacks (10).

In the embodiment shown in FIG. 3, the optional equalization heatexchanger described above is omitted. The equalization heat exchanger isan optional component and may assist in minimizing the temperaturegradients between the stack and the incoming reactants.

A bypass fuel stream (250) may be provided which adds unheated andunreformed fuel to the process fuel stream at a point upstream of theradiative fuel heat exchanger (234). The bypass fuel stream may be usedto adjust the hydrocarbon mix of the fuel utilized by the stacks (10)and to lower the temperature of the fuel entering the stacks (10).

The air source (222) passes into a low temperature air heat exchanger(238) and from there into the high temperature air heat exchanger (242)and then into the radiative air heat exchanger (246). In one embodiment,a bypass air source (243) bypasses both the low and high temperature airheat exchangers and joins the process air stream (222) upstream of theradiative heat exchanger (246). The bypass air source provides a sourceof cool air, which may be used to cool the stack if necessary. Theprocess air stream (222) then passes through the radiative air heatexchanger (246) to produce a heated air stream (248) which may then passto the stacks (10) or the optional equalization heat exchanger (notshown). Control of the bypass air source may provide one means ofeffective control of the stack temperature.

As the heated air stream from the high temperature air heat exchanger isdirected to the radiative heat exchanger (246), unlike the configurationillustrated in FIG. 2, the radiative heat exchanger must of course beconstructed robust enough in terms of materials and design to withstandinput air temperatures which may be 800° C. or higher.

Once reacted in the stacks, the cathode exhaust (50) and the anodeexhaust (52) pass into an afterburner heat exchanger assembly (54) whereany residual fuel in the exhaust stream is combusted as described above.The afterburner combustion stream is then used for heat exchange asdescribed above. One skilled in the art may realize that the hot airstream exiting the high temperature air heat exchanger (242) may be usedto direct radiative energy to the stack from the radiative air heatexchanger (246) which may be useful in startup situations as describedbelow.

In one aspect of the invention, the implementation of the presentinvention may be divided into a hot zone (100) and a low temperaturezone (102). The low temperature zone includes the low temperature airheat exchanger (38, 238) and steam generator or water heat exchanger(26, 226). The hot zone (100) may be further divided into a stack zone(104) and a burner zone (106), which in one embodiment may be anintegrated module. As shown in FIG. 4, the stack zone (104) includes thefuel cell stacks (10), the radiative air heat exchanger (46, 246), theequalization heat exchanger (38) and the secondary reformer (34) orradiative fuel heat exchanger (234). The secondary reformer and theradiative heat exchanger may enclose the stacks (10) and preferably havea large surface area directly exposed to the stacks to capture a maximumamount of heat from the stacks. The stack zone is preferably insulated(108) to minimize ambient heat loss. The burner zone or integratedmodule (106) is also a high temperature zone which includes theafterburner (54), the high temperature air heat exchanger (42) and theprimary reformer (30). The burner zone or integrated module (106) isalso preferably insulated.

In one embodiment, the stack zone is contained in a stack compartment,which is vented through a catalytic converter, which may be electricallyheated for startup, shutdown or other low-temperature operation. Anyfuel gases which leak from the stacks are contained within the stackcompartment and are combusted directly within the stack zone oreliminated by the catalytic converter before being vented to theatmosphere. The equalization heat exchanger (38), if utilized, and astack compartment burner (50) are disposed within the stack compartment.

One embodiment of the stack zone is shown in horizontal cross-section inFIG. 5. The horizontal footprint of the stacks (10) is quadrilateral.Accordingly, the stack enclosure is parallel to the verticallongitudinal axis of the stack and is also quadrilateral. In oneembodment, the radiative air heat exchanger (46, 246) forms three sidesof the enclosure while the secondary reformer (34) or radiative fuelheat exchanger (234) forms the fourth side. The equalization heatexchanger may then be disposed below the stack, which is not shown inFIG. 4 or 5. In other embodiments, the stack enclosure may be circularor another geometry may be used.

With reference to FIG. 5, air enters the radiative air heat exchanger(46, 246) through a vertical manifold (208A) and passes through an outerair flow chamber (200) before reversing direction and passing through anintermediate air flow chamber (202). The air then reverses directionagain and passes through an inner air flow chamber (204) before exitingfrom another vertical manifold (208B) and passing to the stack (10) oran equalization heat exchanger. An insulating layer (206) may beprovided between the intermediate chamber (202) and the inner chamber(204).

Startup of the stacks (10) when the whole system is at an ambienttemperature is initiated by first purging the cathode and exhauststreams, as well as the stack compartment, with air. Process air flow isstarted, primarily through the first stream, thereby avoiding theradiative heat exchanger (46). In one embodiment, a stack compartmentheater (not shown) is provided within the stack zone to provide initialheat to the stacks. The stack compartment heater may be an electricheater or a burner and is typically fired first in a startup procedure.Once the stack compartment heater has raised the stack temperature toabout 200° C., the start burner associated with the afterburner is thenignited.

At this stage, the preferred startup sequence depends on whether ananode purge gas is used or not. Anode purge gas may comprise a mixtureof nitrogen or argon with a small amount of hydrogen, and serves topreserve a reducing atmosphere in the fuel reformers as well as the SOFCanodes as the system heats up. If anode purge gas is used, the purge gasflow is initiated after firing of the stack compartment heater and afterfiring of the startup burner. Once the start burner and the stackcompartment burner have brought up the stack temperature to about 700°C., or slightly below the stack operating temperature, the switch frompurge gas to process fuel may take place.

If no anode purge gas is used and the startup procedure utilizes processfuel and reformate only, then the process fuel flow is initiated afterthe start burner is ignited. For steam reformation type systems, it ispreferred to ensure the steam to carbon ratio of the process fuel flowis within the range of about 1.8 to about 3.5, and more preferablyapproximately 2.6 while at operating temperatures to prevent carbondeposition within the system. The steam to carbon ratio may be ramped upfrom a lower value to the operating value during a startup procedure asthermodynamic properties and equilibrium conditions of the reactantsallow.

The hot exhaust from the start burner heats the primary reformer (30),and both the high temperature and low temperature air heat exchangers(42, 38) as well as the steam generator (26) or water heat exchanger(226). As the primary reformer reaches its operating temperature, andthe air heat exchangers heat the air, the fuel cell stacks beginoperating and the exothermic reactions within the stacks may begin toproduce electricity and heat. In one embodiment, it is preferred duringstartup to direct all or a majority of air flow through the first stream(40) thereby avoiding the radiative air heat exchanger (46). Once thefuel cell stacks (10) approach operating temperatures and beginoperating, sufficient radiative heat is produced to allow use of theradiative heat exchanger (46) as well as the high temperature air heatexchanger (42). Once the fuel cell stacks reach an operatingtemperature, then the stack compartment burner may be shutdown. Thestack compartment burner may be required in low power operations tomaintain adequate stack temperatures.

In an alternative embodiment, radiative heat from the air radiative heatexchanger (246) may be used to supplement or replace the stackcompartment heater in a stack heat up function. The configurationillustrated in FIG. 3 is particularly suited to this task. In thisscenario, hot exhaust from the start burner heats the primary reformer(230), and both the high temperature and low temperature air heatexchangers (242, 238) as well as the steam generator (26) or water heatexchanger (226). As the air radiative heat exchanger (246) receives theheated air stream from the high temperature heat exchanger (242), itheats up sufficiently to heat the stack compartment and the stack byradiative means.

The outlet temperature of the high temperature heat exchanger (242) willexceed 600° C., and may exceed 800° C. In a startup operation, theradiative heat exchanger (246) will therefore experience a largetemperature differential between its inlet and outlet air streams. In astart up situation, the temperature differntial between the inlet andoutlet of the radiative heat exchanger may be as high as 500° C. In oneembodiment, this may be used to advantage because of the thermalbuffering effect (on cathode inlet temperature) of the radiative heatexchanger (246). The stack does not directly experience the same largetemperature differential, but the stack zone receives the same amount ofheat.

Conversely, if the process air stream is being used to cool the stack,the inlet temperature may be 800° C. lower than the outlet temperature.

Furthermore, fast transient situations may be accommodated duringoperation. For example, a considerable amount of heat is extracted fromthe stack by the radiative heat exchanger during full power operationand an effort to lower the air inlet temperature to the radiative heatexchanger must be made, primarily by using the bypass air source (243).A transition to a zero net condition, where the stack requires heatinput, may be accelerated by increasing the air inlet temperature to theradiative heat exchanger.

In any embodiment, it is preferred to provide thermal insulation betweenthe stack zone and the burner zone to permit greater thermal control ofthe system. The temperature of the stack zone may be independentlycontrolled with greater precision by eliminating or reducing conductiveheat transfer from the burner zone to the stack zone.

As will be apparent to those skilled in the art, various modifications,adaptations and variations of the foregoing specific disclosure can bemade without departing from the scope of the invention claimed herein.The various features and elements of the described invention may becombined in a manner different from the combinations described orclaimed herein, without departing from the scope of the invention.

1. A fuel cell system including a fuel cell stack producing an anodeexhaust stream and a cathode exhaust stream, said system comprising: (a)a fuel supply assembly for supplying reformate fuel to the stackcomprising: i. a fuel supply, ii. a water heat exchanger for generatingsteam, iii. a primary reformer, and iv. a radiative fuel heat exchanger;(b) an air supply assembly for supplying air to the stack comprising: i.a low temperature air heat exchanger, ii. a high temperature air heatexchanger, and iii. a radiative air heat exchanger; (c) an afterburnerwhich receives the anode and cathode exhaust streams from the fuel cellstack and combusts the exhaust streams to produce a combustion stream;(d) wherein the combustion stream provides heat energy to the primaryreformer, the high temperature air heat exchanger, the water heatexchanger and the low temperature air heat exchanger; and (e) whereinthe radiative air heat exchanger and radiative fuel heat exchanger eachreceive radiative heat energy from the stack.
 2. The system of claim 1wherein the radiative fuel heat exchanger is a secondary reformer. 3.The system of claim 1 further comprising an equalization heat exchangerwhich receives a reformate supply, the first stream air supply and thesecond stream air supply and outputs a temperature equalized reformatestream and an air stream to the stack.
 4. The system of claim 1 furthercomprising a start heater associated with the afterburner, wherein thestartup heater heats the system downstream from the afterburner.
 5. Thesystem of claim 4 further comprising a stack heater for externallyheating the stack in a stack zone.
 6. The system of claim 1 wherein theair supply assembly comprises a first air stream which passes throughthe high temperature air heat exchanger and a second air stream whichpasses through the radiative air heat exchanger, and wherein the firstair stream and second air stream combine downstream from the radiativeair heat exchanger.
 7. The system of claim 1 wherein the air supplyassembly comprises an air stream which passes through the hightemperature air heat exchanger and the radiative air heat exchanger inseries.
 8. The system of claim 7 wherein the air supply assembly furthercomprises a bypass air stream which does not pass through the hightemperature air heat exchanger.
 9. The system of claim 1 wherein thewater heat exchanger generates steam and mixes the steam with fuel. 10.A fuel cell system including a stack comprising: (a) an fuel supplyassembly for producing a reformate stream comprising a process fuelsupply, a water heat exchanger, a primary reformer, and a radiative fuelheat exchanger; (b) an air supply assembly comprising a low temperatureair heat exchanger, a high temperature air heat exchanger, and aradiative air heat exchanger; (c) an afterburner which receives theanode and cathode exhaust streams from the fuel cell stack and combuststhe exhaust streams to produce a combustion stream; (d) wherein thewater heat exchanger, low temperature air heat exchanger are grouped ina low temperature zone; (e) and wherein the afterburner, hightemperature air heat exchanger and primary reformer are grouped in ahigh temperature zone; (f) and wherein the radiative fuel heatexchanger, radiative air heat exchanger, and fuel cell stack are groupedin a stack zone.
 11. The fuel cell system of claim 10 wherein the hightemperature zone abuts, but is thermally insulated from, each of the lowtemperature and stack zones.
 12. The fuel cell system of claim 11further comprising a stack compartment enclosing the stack, wherein thestack compartment is wholly or partially formed by the radiative fuelheat exchanger and the radiative air heat exchanger.
 13. The fuel cellsystem of claim 12 wherein the radiative fuel heat exchanger comprises asecondary reformer.
 14. The fuel cell system of claim 12 furthercomprising a stack compartment heater for heating the stack compartment.15. The fuel cell system of claim 10 wherein the radiative air heatexchanger comprises at least one planar section having at least twoplanar air flow chambers separated by an insulating layer.
 16. The fuelcell system of claim 15 wherein the radiative heat exchanger comprisesan inner, a middle and an outer planar air flow chamber wherein theinsulating layer separates the inner, air flow chamber and the middleair flow chamber.